Methods for protecting silica-containing article and inhibiting breaks during drawing of optical fiber, and silica-containing article protected against break-inducing particulates

ABSTRACT

A method of protecting a silica-containing article used in the manufacture of an optical fiber includes the step of applying to the silica-containing article a protective layer that facilitates removal of particulates that deposit on the protective layer and that ablates during or can be removed before subsequent processing of the silica-containing article. An intermediate product used in the manufacture of an optical fiber and protected against break-inducing particulates includes a silica-containing article, and a protective layer that facilitates removal of particulates-that have deposited on the protective layer and that can be ablated during or removed before subsequent processing of the intermediate product.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/109,734, filed Nov. 24, 1998.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method of protecting asilica-containing article used in the manufacture of an optical fiber, amethod of inhibiting breaks during drawing of an optical fiber, and anintermediate product used in the manufacture of an optical fiber andprotected against break-inducing particulates.

[0004] 2. Description of the Related Art

[0005] An optical fiber is typically formed by drawing the optical fiberfrom a fiber preform heated to a high temperature. The fiber preform canbe formed by a variety of processes. One such process, which is known asthe outside vapor deposition process, is performed by applying silicasoot to an alumina bait rod to establish a core profile, consolidatingthe core profile to create a glass core blank, and drawing the coreblank to a smaller diameter to create a glass core cane. The core caneis then coated with soot, which is consolidated to create the fiberpreform. Other processes, such as modified vapor deposition (MCVD) orplasma-activated chemical vapor deposition (PCVD), known generally asinside vapor deposition processes, are performed by depositing silica onthe inside of a solid glass tube. The solid glass tube with the depositis then collapsed to form a glass core blank, and glass is added to theoutside of the core blank to form a fiber preform from which the opticalfiber is drawn. Alternatively, the solid glass tube with the deposit canbe collapsed to directly form a fiber preform. Still another processemployed to make a preform for drawing optical fiber is the vapor axialdeposition (VAD) process. The present invention has applicability in atleast all of these various vapor deposition techniques.

[0006] As used herein, the term fiber preform shall refer to an articlefrom which a fiber can be drawn without having to add moresilica-containing glass. Core blank and core cane shall be used to referto articles that include at least part of (but not necessarily all of)the optical core of the resultant fiber. A core cane is a core blankwhich has been consolidated and drawn into a smaller diameter,intermediate product. Thus, in some manufacturing operations, a coreblank (or core cane) may be formed, after which additional core and/orclad glass material will be added to the core blank (or core cane) toform a fiber preform.

[0007] During drawing of an optical fiber from a fiber preform, theoptical fiber often will break. The reduction of breaks during drawingof optical fiber is a clear goal in the industry, especially sincecustomers now request lengths of optical fiber greater than fiftykilometers.

[0008] Fiber breaks are believed to be caused at least in part byinorganic foreign particulates (e.g., ZrO₂) that deposit on glasssurfaces of intermediate products, such as the fiber preform, coreblank, core cane, and glass tube, produced during the formation of theoptical fiber. These glass surfaces are reactive and can formirreversible bonds with the inorganic particulates. As shownschematically in FIG. 2, inorganic particulates 20 bond with activesites, such as OH groups, on a glass surface 10 and become part of theglass surface 10. Therefore, the particulates cannot be readily removedduring standard cleaning before fiber draw. These particulates causestructural failure during fiber draw. For example, inorganicparticulates on the glass surface of the fiber preform, core blank, corecane, or the glass tube, are believed to be a main cause of externalfiber breaks, which occur during the draw process. Inorganicparticulates on the glass surfaces of the core blank, core cane, andglass tube are believed to sometimes cause fiber internal breaks.

[0009] The inorganic particulates are present in the environment of themanufacturing plant. In addition to merely falling unaided onto theglass surfaces of the intermediate products, the particulates may beattracted to the glass surfaces by static charge. Ironically, a staticcharge often develops due to efforts to clean the glass surfaces.

[0010] Particulates can be removed from the glass surfaces of theintermediate products by using hydrofluoric acid as a cleaning agent.Hydrofluoric acid, however, changes the dimensions of the intermediateproduct because it etches the glass surface. Hydrofluoric acid is alsoexpensive to use because it is toxic. Thus, hydrofluoric-acid cleaningis not a desirable technique for reducing fiber breaks.

[0011] It might be possible to reduce fiber breaks by manufacturing in aclean room so that there are almost no particulates to deposit on theglass surfaces of the intermediate products. This, however, would not becost efficient.

SUMMARY OF THE INVENTION

[0012] As embodied and broadly described herein, the invention comprisesa method of protecting a silica-containing article used in themanufacture of an optical fiber and thereby inhibiting breaks duringdrawing of an optical fiber. The method includes the steps of providinga silica-containing article used in the manufacture of an optical fiber,and applying a protective layer to the silica-containing article. Thearticle could be for example, a core blank, a core cane, a fiberpreform, a glass tube used in an inside vapor deposition process, asleeve tube used to build up the glass exterior to the core glass, orany other silica-containing article. Preferably, the silica-containingarticle is a glass (as opposed to unconsolidated silica soot) when theprotective layer is applied.

[0013] The invention also comprises an intermediate product used in themanufacture of an optical fiber and protected against break-inducingparticulates. The intermediate product includes a silica-containingarticle, and a protective layer.

[0014] It is to be understood that the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings, which are hereby incorporated byreference, illustrate an embodiment of the invention and together withthe description serve to explain the principles of the invention.

[0016]FIG. 1 shows a partial schematic, cross-sectional view of a glasssurface coated with a preferred protective layer according to anembodiment of the present invention.

[0017]FIG. 2 shows a partial schematic, cross-sectional view of anuncoated glass surface subjected to inorganic particulates in theenvironment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Reference will now be made in detail to the presently preferredembodiments of the invention.

[0019] It has been determined that fiber breaks during the drawing ofoptical fiber can be reduced by applying a protective layer to varioussilica-containing articles which are made during the course ofmanufacturing an optical fiber. For example, such a protective layer canbe applied to a surface of a silica-containing fiber preform from whichthe optical fiber is drawn. Preferably, the silica-containing article isa silicate based glass article such as an optical fiber preform or aglass article for use in making a optical fiber preform. Such glassarticles are typically comprised of a core region consisting of silicadoped with an index of refraction altering dopant, such as germania orfluorine, and the core is surrounded by a cladding which typicallyconsists of silica or fluorine doped silica. The protective layerprotects the silica-containing article from break-inducing particulates,such as inorganic particulates, and facilitates removal of thoseparticulates prior to drawing of the optical fiber or furtherprocessing. Additionally, the protective layer preferably ablates duringdrawing of the optical fiber or further processing, so that it does notaffect the optical properties of the optical fiber. Alternatively, theprotective layer could be otherwise removed before or during the drawoperation or further processing. The protective layer is preferablyapplied to any glass surface which will see an atmosphere in which itmight come in contact with inorganic particulates. The protective layeris preferably applied to a consolidated, or sintered glass surface, asopposed to an unconsolidated glass soot.

[0020] In particular, the protective layer can be applied to the fiberpreform after the fiber preform is formed. The number of particulatesthat deposit on the fiber preform can be minimized by applying theprotective layer to the fiber preform as soon as possible after it isformed. Just before drawing the optical fiber from the fiber preform,these break-inducing particulates can be removed from the protectivelayer on the fiber preform by, for example, wiping them off with aconventional clean room wipe containing isopropyl alcohol, blowing themoff with super critical CO₂, or rinsing them off with a liquid such aswater, or any other cleaning method suitable for removing theparticulates. Thus, the particulates will not be present on the fiberpreform during drawing of an optical fiber and, therefore, will not bebreak sources in the optical fiber.

[0021] The protective layer preferably facilitates removal of theparticulates by preventing bonding of the particulates to the glasssurface of the fiber preform. In particular, it is believed that theprotective layer preferably bonds to active sites on the glass surfacedue to, for example, a covalent bond, an ionic bond, or a bond due tovan der Waal forces. As shown in FIG. 1, since a protective layer 30bonds to active sites on a glass surface 10, inorganic particulates 20merely rest on the protective layer 30 and do not bond to the activesites. The active sites can include, for example, groups that will forma SiMO_(x) compound, where M is a metal. Examples of groups that willform such a SiMO_(x) compound include OH, SiOH, and GeOH groups.

[0022] The protective layer preferably can be at least partially removedfrom the fiber preform before fiber draw. For example, the protectivelayer can be made in the form of a water soluble polymer such aspolyvinyl alcohol or hydroxymethylcellulose, which can be removed fromthe fiber preform by washing the fiber preform with water or anothersuitable solvent which removes the protective layer.

[0023] Alternatively, in a preferred embodiment, the protective layerablates during drawing of the optical fiber from the fiber preform.Consequently, in the case of a fiber preform, there is no need to removethe protective layer before inserting the preform into the draw furnace.The protective layer should burn off early enough in the drawing processthat it does not become an integral part of the optical fiber. Thetemperature of the furnace during drawing is typically 1400° C. to 2000°C. The protective layer preferably ablates below 900° C. and, morepreferably, below 500° C. (most polymer should burn off below 500° C.,but carbon will burn off between 600-900° C.).

[0024] The protective layer preferably leaves essentially no detrimentalinorganic residue after ablating. As used herein, the term detrimentalinorganic residue refers to residue that will act as break sources. Suchinorganic residue will often not dissolve into the glass and insteadforms part of the glass structure. It is further preferred that theprotective layer does not leave an organic residue or a carboncontaining species.

[0025] It is also preferred that the protective layer prevent the buildup of static on the fiber preform. This prevents particulates from beingattracted to the fiber preform.

[0026] Many materials will provide a protective layer that satisfies theabove-stated desirable criteria of facilitating removal of inorganicparticulates and ablating during drawing of an optical fiber. Many ofthese materials also provide the additional desirable characteristicsstated above.

[0027] For example, many organic materials satisfy the preferredcriteria stated above for the protective layer. In particular, organicmaterials that form a self-assembled monolayer on the silica-containingarticle are presently preferred. Organic materials of this typepreferably have a hydrocarbon or fluorocarbon functionality and includesilane monomers or oligomers. Examples include hydrocarbon silanes,fluorocarbon silanes, epoxy functional silanes, acrylate functionalsilanes, amine functional silanes, thiol functional silanes, phenylfunctional silanes, and any combination of the above. Hydrocarbon silane(e.g., C₁₈H₃₇—Si(OR)₃) and fluorocarbon silane (e.g.,C₃₋₁₀F_(n)—CH₂CH₂—Si(OR)₃) are specific examples of organic materialsthat each meet the preferred requirements stated above.

[0028] Other examples of organic protective layers include alkyl andaryl ammonium compounds, e.g. C₁₈H₃₇N(CH₃)3Cl or C₁₇H₃₅CO₂Na. The formeris presently preferred when the glass is negatively charged, whereas thelatter is preferred when the glass is positively charged.

[0029] Other organic protective layers can bond to the glass via Van derWaal forces. Such examples include acrylate polymers, polyvinyl alcoholand waxes such as ethylenebissteramide.

[0030] Organic material can be applied to the fiber preform by, forexample, mixing the organic material with deionized water or anothersuitable solvent such as isoproponal or acetone for the organic materialand spraying or wiping the solution onto the fiber preform or dippingthe fiber preform into the solution. Preferably, the solution contains0.01% to 2% of the organic material.

[0031] Certain polymers will also satisfy the criteria stated above forthe protective layer. Such polymers include water soluble polymers suchas polyvinyl alcohol or hydroxymethyl cellulose; thermoplastic polymerssuch as polybutylmethacrylate; latex based polymers such as crosslinkedpolybutylmethacrylate latex dispersion in water; thermoset polymers suchas epoxy or urethane; UV curable polymers such as acrylates and epoxies.

[0032] These polymers can be applied by various technologies such as,first dissolving the polymers or monomers in water, as in the case ofpolyvinylalcohol or hydroxy methylcellulose; or in a suitable organicsolvent, such as acetone in the case of polybutylmethacrylate; or byapplying the epoxy, urethane or acrylate monomers or oligomers to theglass surface and subsequently curing these materials via heat or UVlight.

[0033] Additionally, carbon will satisfy the criteria stated above forthe protective layer, particularly for the fiber preform. Carbon can beapplied to the fiber preform by conventional techniques, such as vapordeposition. For example, methane, acetylene, or other carbon compoundscan be decomposed by heating in an inert atmosphere to cause carbon todeposit on the glass surface of the fiber preform.

[0034] The following example illustrates an advantage of the invention.

EXAMPLE 1

[0035] A C18-hydrocarbon silane (HC-silane) coating (DuPont, TLF-8291)was prepared as 1% solution in water. Several fiber preforms werecoated, then allowed to stand in a plant environment for several hours.The fiber preforms were then wiped with a clean room cloth containingisopropyl alcohol before being drawn into optical fiber.

[0036] The cloth containing isopropyl alcohol only removed the dirt thatattached to the protective layer during exposure to the plantatmosphere. The cloth containing isopropyl alcohol did not remove thesilane protective layer. Instead, the silane layer ablated during thefiber draw process due to the high temperatures employed. TOF-SIMSanalysis performed on the fiber indicated residual silane left on thedrawn fiber product, and no deleterious effects were seen on theproperties of the fiber due to using this protective coating process. Onthe other hand, using the protective silane coatings described above,break rates were significantly reduced compared to break rates normallyexpected with this type of fiber.

[0037] For ease of explanation, the present invention has been describedwith reference to providing a protective layer on a fiber preform toform an intermediate product protected against break-inducingparticulates. A protective layer, however, can also be provided on othersilica-containing articles used in the manufacture of optical fibers, toform intermediate products protected against break-inducingparticulates. For example, the silica-containing article could be a corecane and/or a core blank used in an outside vapor deposition process. Asan additional example, the silica-containing article could be a glasstube used in an inside vapor deposition process.

[0038] In each of these instances, the protective layer is preferablyapplied to a consolidated or sintered (as opposed to soot) glass surfaceon the silica-containing article, in an amount sufficient to provide acoating which is sufficiently continuous that it protects the articleagainst particulates which would otherwise become potential sources offiber breaks during the fiber draw process. These protective layers arepreferably at least applied onto areas which are critical to preventingbreaks caused by foreign particulates. An example of such a criticalarea is the area on a core cane onto which additional core or clad glassor glass soot is to be added. By employing a protective layer whichcompletely or substantially covers at least the critical areas of thesilica containing article, breaks due to inorganic particulates comingin contact with the silica-containing article can be reducedsubstantially.

[0039] The protective layer can be removed from these additionalintermediate products via techniques similar to those set forth abovefor the fiber preform, e.g., they may be removed partially or completelyprior to further processing using a solvent. Alternatively, in apreferred embodiment which employs the same silane employed in Example 1above to protect a glass core cane, the material ablates duringdeposition of additional core or clad glass soot via an outside vapordeposition process.

[0040] The protective layer applied to these silica-containing articlespreferably meets the criteria stated above in regard to the protectivelayer for the fiber preform. The point in the manufacturing process atwhich the protective layer ablates or is removed from thesesilica-containing articles may differ.

[0041] For example, if a protective layer applied to a silica-containingarticle other than a fiber preform is intended to ablate, it preferablyablates before drawing of the optical fiber and, more preferably,ablates during subsequent processing of the article. For example, in anoutside vapor deposition process, it would be advantageous for aprotective layer on a core blank to ablate during drawing of the coreblank into a core cane. Likewise, it would be advantageous for aprotective layer on a core cane to ablate during the deposition of sooton the core cane. The protective coatings described herein exhibitexcellent utility in protecting core cane or other intermediate glassarticles before additional glass soot is added to such articles. In suchembodiments, the protective coating may be applied onto a glass corecane to protect the core cane until the core cane is to be furtherprocessed. Then, when additional soot material is added onto the corecane, the core cane is preferably first heated to remove the organicprotective coating, after which additional glass soot is deposited ontothe core cane. With regard to the inside vapor deposition process, as afurther example, it would be advantageous for a protective layer on aglass tube to ablate during fire polishing or other tube preparatorysteps employed just prior to deposition, or just as the first layer ofadditional soot is being deposited.

[0042] Likewise, if a protective layer is intended to be removed from acore blank, core cane, or glass tube, it preferably is removed before orduring subsequent processing, such as redraw or the deposition ofadditional soot.

[0043] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the above-describedembodiments of the present invention without departing from the scope orspirit of the invention. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A method of protecting a silica-containingarticle used in the manufacture of an optical fiber, the methodcomprising the steps of: providing a silica-containing article used inthe manufacture of an optical fiber; and applying a protective layer tothe silica-containing article.
 2. The method of claim 1, wherein theprotective layer is applied to a consolidated glass surface.
 3. Themethod of claim 2, wherein the protective layer ablates duringsubsequent processing of the silica-containing article.
 4. The method ofclaim 2, wherein the protective layer leaves essentially no detrimentalinorganic residue after ablating.
 5. The method of claim 2, wherein theprotective layer inhibits bonding of particulates to thesilica-containing article.
 6. The method of claim 5, wherein theprotective layer inhibits bonding by occupying active sites on thesilica-containing article such that particulates cannot bond to thoseactive sites.
 7. The method of claim 6, wherein the active sites includegroups that will form a SiMO_(x) compound, where M is a metal.
 8. Themethod of claim 2, wherein the protective layer includes carbon.
 9. Themethod of claim 2, wherein the protective layer includes an organicmaterial.
 10. The method of claim 9, wherein the protective layerincludes at least one of a water soluble polymer, a thermoplasticpolymer, a latex based polymer, a thermoset polymer, and a UV curablepolymer.
 11. The method of claim 9, wherein the organic material forms aself-assembled monolayer on the silica-containing article.
 12. Themethod of claim 9, wherein the organic material includes at least one ofhydrocarbon silane, fluorocarbon silane, epoxy functional silanes,acrylate functional silane, amine functional silane, thiol functionalsilane, phenyl functional silane, an alkyl and aryl ammonium compound,acrylate polymer, polyvinyl alcohol, and a wax.
 13. The method of claim2, further comprising the step of removing particulates from theprotective layer.
 14. The method of claim 2, further comprising the stepof removing the protective layer from the silica-containing articleduring further processing.
 15. The method of claim 2, further comprisingthe step of removing the protective layer from the silica-containingarticle before fiber draw.
 16. The method of claim 2, wherein thesilica-containing article includes one of a core cane and a core blankused in an outside vapor deposition process.
 17. The method of claim 2,wherein the silica-containing article includes a glass tube used in aninside vapor deposition process.
 18. The method of claim 2, wherein thesilica-containing article is a fiber preform from which an optical fibercan be drawn and the protective layer is applied directly onto the fiberpreform.
 19. The method of claim 18, further comprising the step ofdrawing an optical fiber from the fiber preform.
 20. The method of claim19, wherein the protective layer ablates during drawing of an opticalfiber from the fiber preform.
 21. The method of claim 20, wherein theprotective layer leaves essentially no detrimental inorganic residueafter ablating.
 22. The method of claim 18, wherein the protective layerinhibits bonding of particulates to the fiber preform.
 23. The method ofclaim 22, wherein the protective layer inhibits bonding by occupyingactive sites on the fiber preform such that particulates cannot bond tothose active sites.
 24. The method of claim 23, wherein the active sitesinclude groups that will form a SiMO_(x) compound, where M is a metal.25. The method of claim 18, wherein the protective layer includescarbon.
 26. The method of claim 18, wherein the protective layerincludes an organic material.
 27. The method of claim 26, wherein theprotective layer includes at least one of a water soluble polymer, athermoplastic polymer, a latex based polymer, a thermoset polymer, and aUV curable polymer
 28. The method of claim 26, wherein the organicmaterial forms a self-assembled monolayer on the fiber preform.
 29. Themethod of claim 26, wherein the organic material includes at least oneof hydrocarbon silane, fluorocarbon silane, epoxy functional silanes,acrylate functional silane, amine functional silane, thiol functionalsilane, phenyl functional silane, an alkyl and aryl ammonium compound,acrylate polymer, polyvinyl alcohol, and a wax.
 30. The method of claim18, further comprising the step of removing particulates from theprotective layer.
 31. The method of claim 18, further comprising thestep of removing the protective layer from the fiber preform beforefiber draw.
 32. The method of claim 18, wherein the fiber preform isformed by adding additional soot materials by an outside vapordeposition process onto a core and a core blank, the method furthercomprising the steps of applying a protective layer to at least one ofthe core cane and the core blank and removing particulates from theprotective layer on the at least one of the core cane and the coreblank.
 33. The method of claim 18, wherein the fiber preform is formedby an inside vapor deposition process from a silica-containing tube, themethod further comprising the steps of applying a protective layer tothe silica-containing tube and removing particulates from the protectivelayer on the silica-containing tube.
 34. An intermediate product used inthe manufacture of an optical fiber and protected against break-inducingparticulates, the intermediate product comprising: a silica-containingarticle; and a protective layer.
 35. The intermediate product of claim34, wherein the protective layer can be removed before subsequentprocessing of the intermediate product.
 36. The intermediate product ofclaim 34, wherein the protective layer can be ablated during subsequentprocessing of the intermediate product.
 37. The intermediate product ofclaim 36, wherein the protective layer leaves essentially no detrimentalinorganic residue after ablating.
 38. The intermediate product of claim34, wherein the protective layer inhibits bonding of particulates to thesilica-containing article.
 39. The intermediate product of claim 38,wherein the protective layer inhibits bonding by occupying active siteson the silica-containing article such that particulates cannot bond tothose active sites.
 40. The intermediate product of claim 39, whereinthe active sites include groups that will form a SiMO_(x) compound,where M is a metal.
 41. The intermediate product of claim 38, whereinthe protective layer includes carbon.
 42. The intermediate product ofclaim 38, wherein the protective layer includes an organic material. 43.The intermediate product of claim 42, wherein the protective layerincludes at least one of a water soluble polymer, a thermoplasticpolymer, a latex based polymer, a thermoset polymer, and a UV curablepolymer.
 44. The intermediate product of claim 42, wherein the organicmaterial forms a self-assembled monolayer on the silica-containingarticle.
 45. The intermediate product of claim 42, wherein the organicmaterial includes at least one of hydrocarbon silane, fluorocarbonsilane, epoxy functional silanes, acrylate functional silane, aminefunctional silane, thiol functional silane, phenyl functional silane, analkyl and aryl ammonium compound, acrylate polymer, polyvinyl alcohol,and a wax.
 46. The intermediate product of claim 34, wherein thesilica-containing article includes a fiber preform from which an opticalfiber is drawn.
 47. The intermediate product of claim 34, wherein thesilica-containing article includes one of a core cane and a core blankused in an outside vapor deposition process.
 48. The intermediateproduct of claim 34, wherein the silica-containing article includes aglass tube used in an inside vapor deposition process.
 49. Theintermediate product of claim 34, wherein the protective layer isapplied to a consolidated glass surface.